The polychlorinated dibenzo-p-dioxins/dibenzofurans 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) were studied for liver tumour promoting activity in a medium-term altered foci assay in nitrosamine-initiated female Sprague-Dawley rats. The congeners under study were administered by weekly subcutaneous injections at three dose levels for 20 weeks. Evaluation of gamma-glutamyltranspeptidase (GGT+), altered hepatic foci development, showed that all congeners studied acted as potent promoters of hepatocarcinogenesis. TCDD and PeCDD were virtually equipotent as enhancers of foci development while PeCDF displayed approximately ten per cent of the activity of the dioxins. Analysis of the dioxin- and furan-congeners by gas chromatography/mass spectroscopy (GC/MS) technique showed that the retention of PeCDD and PeCDF in liver tissue was approximately 7 and 20 times, respectively, as high as the retention of TCDD. Based on the concentration of the respective congener in liver tissue, PeCDD and PeCDF were 0.14 and 0.007 times as active as TCDD as promoters of foci development. The dose related enhancement of GGT+ foci development induced by the PCDD/PCDF congeners was accompanied by an increased incidence of histological changes in the liver.